Case-hardened steel component

ABSTRACT

A case-hardened steel component includes a base metal and a carburized layer, the base metal has a predetermined chemical composition, a number density of Ti-based precipitates having an equivalent circle diameter of 5 to 50 nm in a surface region from a surface to a depth of 0.1 mm is 0.5 pieces/μm 2  or more, and in the surface region, a number density of AlN having an equivalent circle diameter of 50 nm or more and 100 nm or less is 0.5 pieces/μm 2  or less.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a case-hardened steel component.

Priority is claimed on Japanese Patent Application No. 2015-070701,filed on Mar. 31, 2015, the content of which is incorporated herein byreference.

RELATED ART

There may be cases where a component for machine structural use isbroken by suddenly receiving high stress. In particular, in a gear for avehicle such as a differential gear, a transmission gear, and acarburized shaft with a gear, the root of the tooth may be broken byimpact fracture due to a load during sudden acceleration and sudden stopof the vehicle. In order to prevent such a phenomenon, particularly in adifferential gear and a transmission gear, the improvement in the impactvalue thereof (impact resistance) is more desirable. By sufficientlyimproving the impact value of such a component for machine structuraluse, the amount of the materials used for the component for machinestructural use can be decreased, and a reduction in the weight of thecomponent for machine structural use can be achieved.

It is generally known that grain refining is effective in improving theimpact value. Grain refining can be realized by forming a large amountof fine precipitates in steel.

A technique in the related art for precipitating a large amount of fineprecipitates in steel will be described below. For example, PatentDocument 1 proposes a technique for dispersing AlN or AlN and Nb(CN) insteel in order to prevent grain coarsening during carburizing. However,since AlN has low thermal stability, AlN is easily solutionized in steeland coarsened, thereby forming coarser precipitates than otherprecipitates formed in the steel. Therefore, AlN has an insufficienteffect on grain refining. In addition, Nb necessary for forming Nb(CN),while being capable of forming fine Nb(CN) in steel, reduces the hotductility of steel and causes the occurrence of flaws during casting andhot rolling. Therefore, it is not preferable to include a large amountof Nb.

Patent Document 2 proposes a technique for obtaining fine AlN duringcarburizing by reducing the amount of AlN precipitated after hotforging. However, since AlN has low thermal stability and is easilycoarsened, the grain refining effect is insufficient.

As described above, the techniques disclosed in Patent Documents 1 and 2cannot satisfactorily meet the demand for a case-hardened steelcomponent having an excellent impact value and high hot ductility.

PRIOR ART LITERATURE Patent Literature

[Patent Document 1] Japanese Patent No. 3738004 [Patent Document 2]Japanese Patent No. 3954772

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In view of the above problems, an object of the present invention is toprovide a case-hardened steel component which is excellent in impactresistance without impairing manufacturability.

Means for Solving the Problem

The gist of the present invention is as follows.

(1) According to an aspect of the present invention, a case-hardenedsteel component includes: a base metal; and a carburized layer, in whicha chemical composition of the base metal includes, by mass %, C: 0.16%to 0.30%, Si: 0.10% to 2.00%, Mn: 0.30% to 2.00%, Cr: 0.20% to 3.00%, S:0.001% to 0.100%, N: 0.003% to 0.010%, Ti: 0.020% to 0.200%, Nb: 0.010%or more and less than 0.050%, B: 0.0005% to 0.0050%, Ni: 0% to 3.00%,Mo: 0% to 0.80%, Cu: 0% to 1.00%, Co: 0% to 3.00%, W: 0% to 1.00%, V: 0%to 0.30%, Pb: 0% to 0.50%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0%to 0.0500%, Te: 0% to 0.1000%, a rare earth metal: 0% to 0.0050%, Al:limited to 0.010% or less, O: limited to 0.0050% or less, P: limited to0.025% or less, and Fe and impurities as a remainder, a number densityof Ti-based precipitates having an equivalent circle diameter of 5 to 50nm in a surface region from a surface to a depth of 0.1 mm is 0.5pieces/μm² or more, and in the surface region, a number density of AlNhaving an equivalent circle diameter of 50 nm or more and 100 nm or lessis 0.5 pieces/m² or less.

(2) In the case-hardened steel component according to (1), the chemicalcomposition of the base metal may include, by mass %, one or two or moreselected from the group consisting of Ni: more than 0% and 3.00% orless, Mo: more than 0% and 0.80% or less, Cu: more than 0% and 1.00% orless, Co: more than 0% and 3.00% or less, W: more than 0% and 1.00% orless, and V: more than 0% and 0.30% or less.

(3) In the case-hardened steel component according to (1) or (2), thechemical composition of the base metal may include, by mass %, one ortwo or more selected from the group consisting of Pb: more than 0% and0.50% or less, Ca: more than 0% and 0.0100% or less, Mg: more than 0%and 0.0100% or less, Zr: more than 0% and 0.0500% or less, Te: more than0% and 0.1000% or less, and a rare earth metal: more than 0% and 0.0050%or less.

Effects of the Invention

According to the aspect of the present invention, it is possible toprovide a case-hardened steel component having excellent impactresistance. The case-hardened steel component causes a reduction in theamount of materials used for a component for machine structural use andcontributes to a reduction in the weight of the component for machinestructural use. Therefore, the industrial effect of the presentinvention is extremely large.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of the carburized layer of a case-hardenedsteel component according to an embodiment.

FIG. 2 is a conceptual view of the carburized layer of a case-hardenedsteel component obtained from steel having an excessive amount of Al.

FIG. 3 is a conceptual view of the carburized layer of a case-hardenedsteel component obtained by an inappropriate carburizing treatment.

FIG. 4 is a graph showing a temperature change during a carburizingtreatment.

FIG. 5 is a side view of a test piece used to evaluate an impact valueratio.

EMBODIMENTS OF THE INVENTION

The inventors examined a method of realizing a case-hardened steelcomponent having excellent impact resistance by allowing precipitateshaving high thermal stability to be present on the surface layer of asteel component without impairing manufacturability. Specifically, gascarburizing and an impact test were conducted on steel widely andsystematically varied in chemical composition and carburizing materialproperties. As a result, the following findings were obtained.

The inventors investigated the relationship between the grain size aftercarburizing and precipitates in steel. As a result, it was found thatthe factors that control the effect of refining grain sizes aftercarburizing by precipitates in the steel are the distribution state ofthe precipitates at the time of reverse transformation to γ duringheating in gas carburizing, that is, at the time of raising atemperature to 700° C. to 800° C., and the stability of the precipitatesat the time of temperature retention during carburizing heating.

Furthermore, the inventors found that N₂ gas contained in thecarburizing gas atmosphere decomposes during heating in gas carburizing,and N is introduced into the steel and affects the behavior duringreverse transformation to γ. In order to investigate a method ofrefining grains by utilizing the introduced N, the inventors conductedintensive investigations on the influence of chemical compositions andcarburizing heating conditions. As a result, the inventors found that ina case where Al is contained in steel in an amount of more than 0.010%,the introduced N reacts with Al to form coarse AlN, and in a case wherethe Al content in the steel is 0.010% or less and the Ti content in thesteel is 0.020% or more, N introduced into the steel does not formcoarse AlN and thus grains are refined, leading to the improvement in animpact value.

The inventors presumed that the above-described phenomenon occursbecause MN hinders the grain refining effect of Ti-based precipitatessuch as TiC, (Ti,Nb)(C,N), TiN, and Ti₄C₂S₂, particularly Ti-basedprecipitates primarily formed of Ti and N. Furthermore, the inventorsfound that in a case where the average temperature rising rate in arange of 700° C. to 800° C. is 5 to 50° C./min, the diameter and numberdensity of the Ti-based precipitates and coarse AlN are optimized andthus the grains can be refined.

In addition, the inventors found that a reduction in the hot ductilitydue to Nb can be suppressed by limiting the Nb content to less than0.050% and setting the Ti content to 0.020% or more.

Hereinafter, a case-hardened steel component according to an embodimentof the present invention based on the above-described findings (acase-hardened steel component according to this embodiment) will bedescribed in detail.

The case-hardened steel component according to this embodiment includesa base metal and a carburized layer. The case-hardened steel componentaccording to this embodiment is obtained by processing steel having apredetermined chemical composition and performing gas carburizing on thesteel.

The reason for limiting the chemical composition of the base metal ofthe case-hardened steel component according to this embodiment will bedescribed. The chemical composition of the base metal is the same asthat of the steel before processing. Hereinafter, “%” which is a unitassociated with the amount of an alloying element represents “mass %”.

(C: 0.16% to 0.30%)

The C content determines the strength of the core part (base metal) ofthe case-hardened steel component and also affects the effective casedepth. In order to secure the required core part strength, the lowerlimit of the C content is set to 0.16%. On the other hand, when the Ccontent is excessive, the efficiency of cutting and cold workingdecreases. Therefore, the upper limit of the C content is set to 0.30%.A preferable lower limit of the C content is 0.18%, and a morepreferable lower limit thereof is 0.19%. A preferable upper limit of theC content is 0.26%, and a more preferable upper limit thereof is 0.24%.

(Si: 0.10% to 2.00%)

Si is an element effective in deoxidizing steel and is an elementeffective in improving the hardenability of the steel and securing thestrength required for a component for machine structural use. When theSi content is less than 0.10%, the effect is insufficient. On the otherhand, when the Si content exceeds 2.00%, decarburization at the time ofproduction becomes significant and the strength and effective case depthof the carburized steel component (case-hardened steel component) becomeinsufficient. For the above reasons, it is necessary to set the Sicontent to be in a range of 0.10% to 2.00%. A preferable lower limit ofthe Si content is 0.15%, and a more preferable lower limit thereof is0.20%. A preferable upper limit of the Si content is 1.00%, and a morepreferable upper limit thereof is 0.50%.

(Mn: 0.30% to 2.00%)

Mn is an element effective in deoxidizing steel and is an elementeffective in increasing the hardenability and imparting necessarystrength to the steel. Mn combines with S to form MnS in the steel andhas an effect of improving the machinability. When the Mn content isless than 0.30%, this effect is insufficient. On the other hand, whenthe Mn content exceeds 2.00%, the retained austenite is stably presentin the steel, and the strength of the steel decreases. For the abovereasons, it is necessary to set the Mn content to be in a range of 0.30%to 2.00%. A preferable lower limit of the Mn content is 0.35%, and amore preferable lower limit thereof is 0.40%. A preferable upper limitof the Mn content is 1.50%, and a more preferable upper limit thereof is1.20%.

(Cr: 0.20% to 3.00%)

Cr is an element effective in increasing the hardenability and impartingthe necessary strength to steel. When the Cr content is less than 0.20%,the effect is insufficient. On the other hand, when the Cr contentexceeds 3.00%, the effect is saturated. For the above reasons, it isnecessary to set the Cr content to be in a range of 0.20% to 3.00%. Apreferable lower limit of the Cr content is 0.50%, and a more preferablelower limit thereof is 0.80%. A preferable upper limit of the Cr contentis 2.20%, and a more preferable upper limit thereof is 2.00%.

(S: 0.001% to 0.100%)

S forms MnS in steel, thereby improving the machinability of the steel.When the S content is less than 0.001%, the effect is insufficient. Onthe other hand, when the S content exceeds 0.100%, not only is theeffect saturated, but also S is segregated to grain boundaries andcauses boundary embrittlement. For the above reasons, it is necessary toset the S content to be in a range of 0.001% to 0.100%. A preferablelower limit of the S content is 0.010%, and a more preferable lowerlimit thereof is 0.013%. A preferable upper limit of the S content is0.050%, and a more preferable upper limit thereof is 0.040%.

(N: 0.003% to 0.010%)

N bonds to Ti, Nb, V, and the like in steel to form coarse nitridesduring solidification in a steel casting process. These coarse nitridesact as fracture origins when present in a large amount. Therefore, it isnecessary to reduce the N content to 0.010% or less. The Ti-basedprecipitates included in the surface part of the case-hardened steelcomponent according to this embodiment are primarily formed by Nintroduced into the steel during the carburizing treatment, and are notformed by N contained in the steel before the carburizing treatment.Therefore, it is not necessary for the base metal of the case-hardenedsteel component according to this embodiment to contain N. However,reducing the N content to less than 0.003% increases manufacturingcosts. Therefore, the lower limit of the N content may be 0.003%. Thelower limit of the N content may be 0.0035% or 0.004%. A preferableupper limit of the N content is 0.009%, and a more preferable upperlimit thereof is 0.008%.

(Ti: 0.020% to 0.200%)

Ti produces Ti-based precipitates such as fine TiC, TiN, Ti(CN), andTiCS in steel and contributes to grain refining. In addition, Ti bondsto C and N introduced into the steel during the carburizing treatment toproduce Ti-based precipitates primarily formed of Ti, C, and/or N, andthus contributes to grain refining. Furthermore, Ti has an effect ofimproving a reduction of the hot ductility due to Nb. These effects areinsufficient when the Ti content is less than 0.020%. On the other hand,when the Ti content exceeds 0.200%, the effects are saturated. For theabove reasons, it is necessary to set the Ti content to be in a range of0.020% to 0.200%. A preferable lower limit of the Ti content is 0.025%,and a more preferable lower limit thereof is 0.030%. A preferable upperlimit of the Ti content is 0.180%, and a more preferable upper limitthereof is 0.160%.

(Nb: 0.010% or More and Less than 0.050%)

Nb is solutionized into Ti-based precipitates, contributes to anincrease in the amount of precipitates and refining of precipitates, andpromotes grain refining. This effect is insufficient when the Nb contentis less than 0.010%. In order to obtain this effect, the Nb content isset to 0.010% or more. A preferable lower limit of the Nb content is0.012%, and a more preferable lower limit thereof is 0.015%. On theother hand, Nb decreases hot ductility and thus reduces the productivityof steel. A reduction in the hot ductility due to Nb can be suppressedby including 0.020% or more of Ti like the base metal of the steelaccording to this embodiment. However, even when the Ti content is0.020% or more, if the Nb content is 0.050% or more, the hot ductilityof the steel decreases, which causes flaws during casting and hotrolling. Therefore, it is necessary to set the Nb content to be lessthan 0.050%. A preferable upper limit of the Nb content is 0.040%, and amore preferable upper limit thereof is 0.030%.

(B: 0.0005% to 0.0050%)

B has a function of suppressing boundary segregation of P. B also has aneffect of improving intergranular strength and intragranular strengthand an effect of improving hardenability. These effects consequentlyimprove the impact value of the steel. These effects cannot besufficiently obtained when the B content is less than 0.0005%. On theother hand, when the B content exceeds 0.0050%, the effects aresaturated. For the above reasons, the B content is set to be in a rangeof 0.0005% to 0.0050%. The lower limit of the B content is preferably0.0010%, and more preferably 0.0013%. The upper limit of the B contentis preferably 0.0045%, and more preferably 0.0040%.

(Al: 0.010% or Less)

Al usually becomes MN and precipitates in steel. However, in thecase-hardened steel component according to this embodiment, Al coexistswith Ti to form a solid solution in the steel. The Al content (that is,the Al content of the base metal) of the steel before the carburizingtreatment significantly influences the properties of the carburizedlayer. As shown in FIG. 1, a carburized layer 1 of the case-hardenedsteel component according to this embodiment has fine Ti-basedprecipitates 2 having a diameter (equivalent circle diameter) of 5 to 50nm. The Ti-based precipitates 2 are formed by C and N introduced intothe steel from the carburizing gas atmosphere during heating in gascarburizing. However, when the Al content of the steel before thecarburizing treatment exceeds 0.010%, as shown in FIG. 2, during heatingin gas carburizing, N and Al introduced from the carburizing gasatmosphere combine together to form relatively coarse AlN 3. In the casewhere the coarse AlN 3 is formed, the grain refining effect cannot besufficiently obtained. Therefore, it is necessary to limit the Alcontent to 0.010% or less. The upper limit of the Al content ispreferably 0.009%, and more preferably 0.007%. Since it is preferablethat the Al content is small, the lower limit of the Al content is 0%.

(O: 0.0050% or Less)

O forms oxides in steel. There may be cases where the oxides undergoboundary segregation and cause boundary embrittlement. In addition, O isan element which forms hard oxide-based inclusions in steel and makes iteasier for brittle fracture to occur. Therefore, it is necessary tolimit the O content to 0.0050% or less. A preferable upper limit of theO content is 0.0030%, and a more preferable upper limit thereof is0.0025%. Since it is preferable that the O content is small, the lowerlimit of the O content is 0%.

(P: 0.025% or Less)

P segregates to austenite grain boundaries during carburizing, therebycausing boundary fracture. That is, P is an element which decreases theimpact value of a carburized steel component. Therefore, it is necessaryto limit the P content to 0.025% or less. A preferable upper limit ofthe P content is 0.022% or less, and a more preferable upper limitthereof is 0.020%. Since it is preferable that the P content is small,the lower limit of the P content is 0%. However, in a case where P isremoved more than necessary, manufacturing costs increase. Therefore,the substantially lower limit of the P content is usually about 0.004%.

The chemical composition of the base metal of the case-hardened steelcomponent according to this embodiment basically consists of theabove-mentioned elements, Fe, and impurities. However, in order toincrease the hardenability or impact value, instead of a portion of Fe,one or two or more selected from the group consisting of Ni, Mo, Cu, Co,W, and V may be further contained. In addition, instead of a portion ofFe, one or two or more selected from the group consisting of Pb, Ca, Mg,Zr, Te, and rare earth metals (REM) may be contained. However, any ofNi, Mo, Cu, Co, W, V, Pb, Ca, Mg, Zr, Te, and rare earth metals (REM) isan arbitrary element, and may not be contained in the base metal of thecase-hardened steel component according to this embodiment. That is, thelower limit of Ni, Mo, Cu, Co, W, V, Pb, Ca, Mg, Zr, Te and the rareearth metals (REM) is 0%, but may also be more than 0%.

(Ni: 0% to 3.00%)

Ni is an element effective in improving the hardenability and impactvalue of steel. In a case where Ni is contained to obtain theabove-described effect, a preferable lower limit of the Ni content is0.20%, and a more preferable lower limit thereof is 0.50%. On the otherhand, when the Ni content exceeds 3.00%, the retained austenite isstably present in the steel and the strength of the steel decreases.Therefore, even in a case where Ni is contained, the upper limit of theNi content is set to 3.00%. The upper limit of the Ni content ispreferably 2.00%, and more preferably 1.80%.

(Mo: 0% to 0.80%)

In addition to increasing the hardenability of steel, Mo suppressessegregation of P to grain boundaries. Therefore, Mo is an elementeffective in improving the impact value of the steel. In a case where Mois contained to obtain the above-described effect, a preferable lowerlimit of the Mo content is 0.05%, and a more preferable lower limitthereof is 0.10%. On the other hand, when the Mo content exceeds 0.80%,the effect is saturated. Therefore, even in a case where Mo iscontained, the upper limit of the Mo content is set to 0.80%.

(Cu: 0% to 1.00%)

Cu is an element effective in improving the hardenability of steel. In acase where Cu is contained to obtain the above-described effect, thelower limit of the Cu content is preferably 0.05%, and more preferably0.10%. On the other hand, when the Cu content exceeds 1.00%, the hotductility decreases. Therefore, even in a case where Cu is contained,the upper limit of the Cu content is set to 1.00%.

(Co: 0% to 3.00%)

Co is an element effective in improving the hardenability of steel. In acase where Co is contained to obtain the above-described effect, thelower limit of the Co content is preferably 0.05%, and more preferably0.10%. On the other hand, when the Co content exceeds 3.00%, the effectis saturated. Therefore, even in a case where Co is contained, the upperlimit of the Co content is set to 3.00%.

(W: 0% to 1.00%)

W is an element effective for improving the hardenability of steel. In acase where W is contained to obtain the above-described effect, thelower limit of the W content is preferably 0.05%, and more preferably0.10%. On the other hand, when the W content exceeds 1.00%, the effectis saturated. Therefore, even in a case where W is contained, the upperlimit of the W content is set to 1.00%.

(V: 0% to 0.30%)

V is solutionized into Ti-based precipitates, contributes to an increasein the amount of precipitates and refining of precipitates, and promotesgrain refining. In a case where V is contained to obtain theabove-described effect, a preferable lower limit of the V content is0.10%, and a more preferable lower limit thereof is 0.20%. On the otherhand, when the V content exceeds 0.30%, the precipitates become coarseduring gas carburizing and the grains are coarsened. Therefore, even ina case where V is contained, the upper limit of the V content is set to0.30%.

(Pb: 0% to 0.50%)

Pb is an element which is melted and embrittled during cutting and thusimproves the machinability of steel. In a case where Pb is contained toobtain the above-described effect, a preferable lower limit of the Pbcontent is 0.05%, and a more preferable lower limit thereof is 0.10%. Onthe other hand, when Pb is contained excessively, the manufacturabilitydecreases. Therefore, even in a case where Pb is contained, the upperlimit of the Pb content is set to 0.50%.

(Ca: 0% to 0.0100%)

Ca has an effect of decreasing the melting point of oxides. Since Caoxides are softened by temperature rise during cutting and thus improvethe machinability of the steel. In a case where Ca is contained toobtain the above-described effect, a preferable lower limit of the Cacontent is 0.0003%, and a more preferable lower limit thereof is0.0005%. On the other hand, when the Ca content exceeds 0.0100%, a largeamount of CaS is produced, resulting in a decrease in the machinability.Therefore, even in a case where Ca is contained, the upper limit of theCa content is set to 0.0100%.

(Mg: 0% to 0.0100%)

Mg is a deoxidizing element and produces oxides in steel. Furthermore,Mg-based oxides formed by Mg tend to be a nucleus of crystallizationand/or precipitation of MnS. In addition, Mg sulfides become complexsulfides of Mn and Mg such that MnS is spheroidized. As described above,Mg is an element effective in controlling the dispersion of MnS andimproving the machinability of the steel. In a case of obtaining theseeffects, a preferable lower limit of the Mg content is 0.0005%, and amore preferable lower limit thereof is 0.0010%. On the other hand, whenthe Mg content exceeds 0.0100%, a large amount of MgS is produced,resulting in a decrease in the machinability of the steel. Therefore,even in a case where Mg is contained, the upper limit of the Mg contentis set to 0.0100%. A preferable upper limit of the Mg content is0.0080%, and a more preferable upper limit thereof is 0.0060%.

(Zr: 0% to 0.0500%)

Zr is a deoxidizing element and bonds to O to form oxides. Zr-basedoxides formed by Zr tend to be a nucleus of crystallization and/orprecipitation of MnS. Therefore, Zr is an element effective incontrolling the dispersion of MnS and improving the machinability of thesteel. In a case of obtaining these effects, a preferable lower limit ofthe Zr content is 0.0005%, and a more preferable lower limit thereof is0.0010%. On the other hand, when the Zr content exceeds 0.0500%, theeffect is saturated. Therefore, even in a case where Zr is contained,the upper limit of the Zr content is set to 0.0500%. A preferable upperlimit of the Zr content is 0.0400%, and a more preferable upper limitthereof is 0.0300%.

(Te: 0.1000% or Less)

Te is an element which promotes the spheroidizing of MnS and improvesthe machinability of steel. In a case of obtaining this effect, apreferable lower limit of the Te content is 0.0005%, and a morepreferable lower limit thereof is 0.0010%. On the other hand, when theTe content exceeds 0.1000%, the effect is saturated. Therefore, even ina case where Te is contained, the upper limit of the Te content is setto 0.1000%. A preferable upper limit of the Te content is 0.0800%, and amore preferable upper limit thereof is 0.0600%.

(Rare Earth Metals: 0% to 0.0050%)

Rare earth metals (REM) produce sulfides in steel. The sulfides become anucleus of precipitation of MnS and thus promote the production of MnS,thereby improving the machinability of the steel. In a case of obtainingthis effect, the total amount of the rare earth metals is preferably0.0005%, and more preferably 0.0010%. On the other hand, when the totalamount of the rare earth metals exceeds 0.0050%, the sulfides becomecoarse and decrease the fatigue strength of the steel. Therefore, evenin a case where the rare earth metals are contained, the upper limit ofthe total amount of the rare earth metals is set to 0.0050%. Apreferable upper limit of the total amount of the rare earth metals is0.0040%, and a more preferable upper limit thereof is 0.0030%.

In this embodiment, the rare earth metals collectively refer to 17elements including 15 elements from lanthanum (La) with atomic number 57in the periodic table to lutetium (Lu) with atomic number 71, yttrium(Y), and scandium (Sc). The amount of the rare earth metals means thetotal amount of one or two or more of these elements.

The base metal of the case-hardened steel component according to thisembodiment contains the above-described alloying components and containsFe and impurities as the remainder. Incorporation of elements other thanthe above-mentioned alloying components into the steel from the rawmaterials and manufacturing equipment as impurities is acceptable aslong as the mixing amount thereof is at a level that does not affect theproperties of the steel.

(Carburized Layer)

The case-hardened steel component according to this embodiment includesthe carburized layer formed on the surface part by the gas carburizing.In this embodiment, the carburized layer represents a region in whichthe C content is larger than that of the base metal and the C content is0.60% or more. The gas carburizing mentioned in this embodiment iscarburizing using an endothermic gas generated by mixing a gas such aspropane with air, and the atmosphere during the gas carburizing containsN₂. This gas carburizing does not include gas carbonitriding in whichNH₃ is introduced into the atmosphere.

The depth (effective case depth) of the carburized layer is preferablyat least 0.5 mm or more, more preferably 1.0 mm or more, and forexample, about 1.0 mm.

(Number Density of Ti-Based Precipitates Having Equivalent CircleDiameter of 5 to 50 nm: 0.5 Pieces/μm² or More in Region from Surface toDepth of 0.1 mm)

As shown in FIG. 1, in the carburized layer 1 of the case-hardened steelcomponent according to this embodiment, fine Ti-based precipitates 2having an equivalent circle diameter of 5 to 50 nm are dispersed. Inthis embodiment, the Ti-based precipitates mean precipitates (includingcomplex precipitates) primarily containing Ti, such as TiC,(Ti,Nb)(C,N), TiN, and Ti₄C₂S₂. The Ti-based precipitates having anequivalent circle diameter of 5 to 50 nm exhibit the pinning effect atthe time of phase transformation, and thus have a grain refining effectobtained after the phase transformation. The grain refining improves theimpact value of the steel. In a case where the number density of theTi-based precipitates having an equivalent circle diameter of 5 to 50 nmin the region from the surface to a depth of 0.1 mm is less than 0.5pieces/μm², a case-hardened steel component having a sufficient impactvalue cannot be obtained.

Ti-based precipitates having an equivalent circle diameter of less than5 nm do not affect the properties of the case-hardened steel componentaccording to this embodiment and therefore may not be taken intoconsideration. Ti-based precipitates having an equivalent circlediameter of more than 50 nm (coarse Ti-based precipitates) do not have agrain refining effect, decrease the number density of the Ti-basedprecipitates having an equivalent circle diameter of 5 to 50 nm, andtherefore may not be contained. However, as long as the Ti-basedprecipitates having an equivalent circle diameter of 5 to 50 nm aresufficiently obtained, the Ti-based precipitates having an equivalentcircle diameter of more than 50 nm are allowed to be contained.Therefore, the number density of the Ti-based precipitates having anequivalent circle diameter of more than 50 nm may not be taken intoconsideration. In addition, since it is preferable that the numberdensity of the Ti-based precipitates having an equivalent circlediameter of 5 to 50 nm is large, the upper limit of the number densityof the Ti-based precipitates having an equivalent circle diameter of 5to 50 nm is not particularly specified.

The number density of the Ti-based precipitates having an equivalentcircle diameter of 5 to 50 nm contained in the region (surface region)from the surface to a depth of 0.1 mm can be measured, for example, bythe following means. First, steel is cut perpendicular to the surface ofthe steel component. Next, samples with a region of 7 μm×7 μm, which canbe observed, are extracted by FIB processing from points at a depth of0.02 mm, 0.05 mm, and 0.09 from the surface of the steel component, andthin film samples having a thickness of 100 nm or more and 300 nm orless are prepared. Thereafter, EDS analysis of phases other than Fe,which have an equivalent circle diameter of 5 to 50 nm, is performed onthe sample at each depth position by observing the region of 7 μm×7 μmat a magnification of 200,000-fold using a HAADF-STEM method with afield emission transmission electron microscope, and the number ofphases where Ti is detected among the phases is counted. The valueobtained by dividing the above-described number by the observation areawas used as the number density of Ti-based precipitates at each depthposition and the average of the values can be used as the number densityof the Ti-based precipitates having an equivalent circle diameter of 5to 50 nm contained in the region from the surface of the steel componentto a depth of 0.1 mm.

(Number Density of AlN Having Equivalent Circle Diameter of 50 nm orMore and 100 nm or Less: 0.5 Pieces/μm² or Less)

As described above, C and N are introduced into the steel during thetemperature rising heating in the gas carburizing. In a case where theintroduced N combines with Al solutionized in the steel and AlN isprecipitated, AlN becomes coarse precipitates. In this case, the grainsare not sufficiently refined and the toughness decreases. It is presumedthat this is because AlN hinders the grain refining effect of Ti-basedprecipitates, particularly Ti-based precipitates primarily formed of Tiand N. In order to avoid a decrease in the toughness, coarse AlN havingan equivalent circle diameter of 50 nm or more and 100 nm or less needsto be 0.5 pieces/μm² or less. This feature is obtained by limiting theAl content of the base metal of the steel (that is, the Al content ofthe steel before the carburizing treatment) to 0.010% or less andappropriately controlling the temperature rising rate during thecarburizing.

AlN having an equivalent circle diameter of less than 50 nm does notaffect the properties of the case-hardened steel component according tothis embodiment and may not be taken into consideration. Also, it isbetter not to include AlN having an equivalent circle diameter of morethan 100 nm. However, in a case where the number density of AlN havingan equivalent circle diameter of 50 nm or more and 100 nm or less isappropriately controlled, AlN having an equivalent circle diameter ofmore than 100 nm is not substantially generated. Therefore, the numberdensity of AlN having an equivalent circle diameter of more than 100 nmmay not be taken into consideration. Since it is preferable that thenumber density of AlN having an equivalent circle diameter of 50 nm ormore and 100 nm or less is small, the lower limit of the number densityof AlN having an equivalent circle diameter of 50 nm or more and 100 nmor less is 0 pieces/μm².

The number density of AlN having an equivalent circle diameter of 50 to100 nm contained in the region from the surface to a depth of 0.1 mm canbe measured, for example, by the following means. First, steel is cutperpendicular to the surface of the steel component. Next, samples witha region of 7 μm×7 μm, which can be observed, are extracted by FIBprocessing from points at depths of 0.02 mm, 0.05 mm, and 0.09 from thesurface of the steel component, and thin film samples having a thicknessof 100 nm or more and 300 nm or less are prepared. Thereafter, elementmapping of Al and N of the thin film samples is performed on the sampleat each depth position in a range of 7 μm×7 μm using the field emissiontransmission electron microscope and EDS (energy dispersive X-rayanalysis). In the place where MN is precipitated, the number of Al and Ndetected is significantly higher than that in places where AlN is notprecipitated. Therefore, a region where the number of Al and N detectedis high is determined as AlN, and the number of AlN regions having anequivalent circle diameter of 50 nm or more and 100 nm or less iscounted. By dividing the above-described number by the observation area,the number density of AlN having an equivalent circle diameter of 50 to100 nm at each depth position is obtained. The average of the values canbe used as the number density of AlN having an equivalent circlediameter of 50 to 100 nm contained in the region from the surface of thesteel component to a depth of 0.1 mm.

Next, a preferable manufacturing method of the case-hardened steelcomponent according to this embodiment will be described. Themanufacturing method of the case-hardened steel component according tothis embodiment includes a step of manufacturing a steel having thechemical composition of the base metal of the case-hardened steelcomponent described above, a step of processing the steel, and a step ofperforming gas carburizing on the steel. The step of manufacturing thesteel and the process of processing the steel are not particularlylimited. However, in order to increase the production efficiency in thestep of processing the steel, it is preferable to manufacture the steelso that the hardness of the steel before processing is low.

In the step of performing gas carburizing, gas carburizing is preferablyperformed under the following conditions.

(Atmosphere of Gas Carburizing: Partial Pressure of N₂ is 40% to 50%)

Gas carburizing performed in the manufacturing of the case-hardenedsteel component according to this embodiment is carburizing using anendothermic gas generated by mixing a gas such as propane with air, andthe atmosphere during the gas carburizing contains N₂. This gascarburizing does not include gas carbonitriding in which NH₃ isintroduced into the atmosphere or vacuum carburizing. The pressure ofthe atmosphere may be substantially the same as the atmosphericpressure. The partial pressure of N₂ in the atmosphere needs to be 40%to 50%. In a case where the partial pressure of N₂ is less than 40%, aninsufficient amount of N is introduced into the steel, and grains cannotbe refined by sufficiently producing Ti-based precipitates having anequivalent circle diameter of 5 to 50 nm. On the other hand, in a casewhere the partial pressure of N₂ exceeds 50%, N is excessivelyintroduced into the steel, Ti-based precipitates become coarse, and theamount of Ti-based precipitates having an equivalent circle diameter of5 to 50 nm is insufficient. Therefore, grains cannot be refined.

(Temperature Rising Rate During Gas Carburizing: 5° C./min to 50° C./minin Temperature Range of 700° C. to 800° C.)

At the time of gas carburizing, as shown in FIG. 4, the steel (steelcomponent material) is heated to a carburizing temperature, retained ata constant temperature in the carburizing temperature, and then cooled.In the temperature rising process during the gas carburizing, C and N₂in the carburizing gas atmosphere are introduced into the steel, and atthe same time, reverse transformation to γ occurs in the steelstructure. The introduced C and N affect the precipitation of AlN,Ti-based precipitates, and the like, and thus affect the grain size. Theamount of C and N introduced is affected by the temperature rising rate,and is mainly dominated by the average temperature rising rate in 700°C. to 800° C. where the reverse transformation to γ (austenite) occursin particular. Therefore, in the gas carburizing including themanufacturing method of the case-hardened steel component according tothis embodiment, it is necessary to strictly control the temperaturerising rate in a temperature range of 700° C. to 800° C. (hatched rangein FIG. 4).

Thereafter, during the temperature rising in the carburizing treatment,the temperature of the test piece is measured three times for everyabout 30° C. within the temperature range of 700° C. to 800° C., and thegradient obtained by applying the least squares method to the measuredtemperatures is defined as an “average temperature rising rate in atemperature range of 700° C. to 800° C.”. In the manufacturing method ofthe case-hardened steel component according to this embodiment, in acase where the average temperature rising rate in the range of 700° C.to 800° C. is higher than 50° C./min, the amount of C and N incorporatedis reduced, and the effect of improving the impact value due tostructure refining cannot be obtained. In a case where the averagetemperature rising rate in the range of 700° C. to 800° C. is lower than5° C./min, the grains become coarse and the effect of improving theimpact value cannot be obtained. It is thought that this phenomenonoccurs because the amount of C and N incorporated is so large that theTi-based precipitates 2 become coarse as shown in FIG. 3 and the numberdensity of the Ti-based precipitates having a grain size of 5 to 50 nmis insufficient. Therefore, the upper limit of the average temperaturerising rate in the range of 700° C. to 800° C. during heating in the gascarburizing needs to be 50° C./min, and the lower limit thereof needs tobe 5° C./min. A preferable upper limit of the average temperature risingrate is 40° C./min, and a more preferable upper limit thereof is 35°C./min. A preferable lower limit of the average temperature rising rateis 7° C./min, and a more preferable lower limit thereof is 10° C./min.The temperature rising rate during heating from room temperature to 700°C. is not limited, and may be retained for preheating at less than 700°C.

(Carburizing Temperature During Gas Carburizing: 900° C. to 1050° C.)

(Retention Time During Gas Carburizing: 1 to 10 Hours)

A carburizing temperature T and a retention time t during the gascarburizing affect the thickness of the carburized layer, therebyaffecting the impact value of the case-hardened steel component and thelike. In the manufacturing method of the case-hardened steel componentaccording to this embodiment, it is necessary to set the carburizingtemperature T during the gas carburizing to 900° C. to 1050° C., and itis necessary to set the retention time t to 1 to 10 hours. In a casewhere the carburizing temperature T is lower than 900° C. or in a casewhere the retention time t is shorter than 1 hour, the carburized layeris not sufficiently formed and the hardness as the basic performance ofthe case-hardened steel component is insufficient. The carburized layermentioned here is a region where the C content is larger than that ofthe base metal and the C content is 0.60% or more. On the other hand, ina case where the carburizing temperature T exceeds 1050° C., the damageto the refractory material in a carburizing furnace becomes significantand thus the gas carburizing treatment cannot be performed. In addition,when the retention time is prolonged, there is a concern thatprecipitates may grow and the grains may become coarse. However, in acase where the retention time is shorter than 10 hours, this problem isnot seen. Therefore, the upper limit of the retention time may be 10hours.

After performing the gas carburizing, it is preferable to performtempering on the case-hardened steel component, for example, at atempering temperature of 150° C. for a tempering time of 90 minutes toensure the toughness of the case-hardened steel component.

Examples

Next, examples of the present invention will be described. Theconditions in the examples are an example of conditions employed toconfirm the applicability and effects of the present invention, and thepresent invention is not limited to the example of conditions. That is,the present invention can employ various conditions without departingfrom the gist of the present invention as long as the object of thepresent invention is achieved.

First, the contents of the examination conducted by the inventors toevaluate the impact resistance of a carburized material will bedescribed below.

First, a steel for carburizing containing C: 0.20 mass %, Si: 0.24 mass%, Mn: 0.79 mass %, P: 0.020 mass %, S: 0.018 mass %, Cr: 1.06 mass %,Al: 0.032 mass %, N: 0.014 mass %, and O: 0.003 mass %, and Fe andimpurities as the remainder was defined as a reference steel. Next, aCharpy impact test piece shown in FIG. 1, which had outer dimensions of10 mm×10 mm×55 mm and had an arcuate notch (notch) having a radius ofcurvature of 10 mm and a depth of 2 mm was defined as a Charpy impacttest piece in this embodiment. The Charpy impact test piece formed ofthe reference steel as its material was heated at a temperature risingrate of 20° C./min in a range of 700° C. to 800° C., was then subjectedto gas carburizing under carburizing conditions (hereinafter, sometimesreferred to as reference carburizing conditions) including a treatmenttemperature (carburizing temperature) of 930° C., a treatment time(retention time) of 2 hours, and a carbon potential of 0.8, and wasfurther subjected to tempering at a tempering temperature of 150° C. fora tempering time of 90 minutes. A Charpy impact test was conducted onthe carburized material, and the absorbed energy thereof was defined asa reference impact value.

The above-mentioned reference steel is a steel having a chemicalcomposition corresponding to SCr420, which is generally used as steelfor a gear, and is the same as the steel of Test No. 15, which will bedescribed later. The gas carburizing performed under the referencecarburizing conditions described above is a general carburizingtreatment performed to manufacture a component for machine structuraluse.

FIG. 5 shows the side surface shape (the shape of the cross sectionperpendicular to the extension direction of the notch) of the Charpyimpact test piece described above. The radius of curvature of the notchis 10 mm and the depth of the notch is 2 mm. The shape of the Charpyimpact test piece is different from the shape of a general Charpy impacttest piece (for example, the shape defined in JIS-Z 2242 “Method forCharpy pendulum impact test of metallic materials”). The Charpy impacttest piece shown in FIG. 5 simulates the root portion of a tooth of agear and is intended to estimate the impact resistance of the rootportion of the tooth when the steel as a test object is machined intothe shape of the gear. As described in, for example, Japanese UnexaminedPatent Application, First Publication No. 2013-40376, this Charpy impacttest piece is widely used as the shape of a test piece for measuring theimpact resistance of carburized steel. The Charpy absorbed energy wasmeasured on the basis of JIS-Z 2242 “Method for Charpy pendulum impacttest of metallic materials” except for the shape of the Charpy impacttest piece. The temperature at which the Charpy impact test wasconducted was set to 25° C. The Charpy impact test piece was prepared bymachining.

Furthermore, the value obtained by dividing the Charpy absorbed energyat 25° C. of the carburized material obtained by performing carburizingand tempering on the Charpy impact test pieces manufactured undervarious conditions by the reference impact value was defined as animpact value ratio under the conditions.

Since a case-hardened steel component having an impact value ratio of1.20 or more has sufficiently improved impact resistance, by applyingthe case-hardened steel component having an impact value ratio of 1.20or more, the design of a component can be changed to ensure impactfracture resistance while suppressing the amount of materials used. Inthe technical field of the machine structural component, it isconsidered that in order to implement such design changes, the impactvalue needs to be improved by 20% with respect to the reference impactvalue described above (the impact value of SCr420 carburized undergeneral carburizing conditions). Therefore, in the present invention, itwas determined that when the impact value ratio was 1.20 or more, theimpact resistance was excellent.

Based on the above-described method, the inventors manufactured testpieces simulating the case-hardened steel component under variousconditions and evaluated the impact resistance.

Specifically, first, various steel ingots having chemical compositionsshown in Tables 1 and 2 were hot-forged to a diameter of 35 mm. Theheating temperature of the hot forging was 1250° C. Thereafter, thesteel ingots were retained at 950° C. for 2 hours, thereafter subjectedto a normalizing treatment under air cooling conditions, and machinedinto the shape of the Charpy impact test piece, shown in Table 1, havingouter dimensions of 10 mm×10 mm×55 mm and an arcuate notch having aradius of curvature of 10 mm and a depth of 2 mm. The shape of this testpiece is the same as that of the Charpy impact test piece shown in FIG.5. Next, this Charpy impact test piece was subjected to a carburizingtreatment. In the carburizing treatment, heating to 930° C. at atemperature rising rate shown in Table 3 was performed. Regarding thetemperature rising rate, the temperature of the test piece was measuredthree times using a radiation-type thermometer in a range of 700° C. to800° C. during the temperature rising, and the gradient obtained by theleast squares method was used as the temperature rising rate. The carbonpotential was set to 0.8, and after retention at a carburizingtemperature for 2 hours, quenching was performed in an oil at 130° C.Tempering was performed under conditions including a temperingtemperature of 150° C. and a tempering time of 90 minutes. After thetempering, a Charpy impact test was conducted to measure the Charpyabsorbed energy (impact value). The Charpy impact test was conductedaccording to the method defined in JIS-Z 2242 except for the shape ofthe notch of the Charpy impact test piece. The test temperature was setto 25° C.

Furthermore, by dividing the impact value of each sample by the impactvalue of Test No. 15, the impact value ratio of each sample wascalculated. The steel of Test No. 15 is the above-mentioned referencesteel.

In addition, using the test piece after the Charpy impact test, thenumber of precipitated AlN from the surface of each sample to a depth of0.1 mm was measured. The measurement method is as follows. First, inorder to obtain the cross section of the notch bottom, the test piecewas cut in a cross section which was perpendicular to the notch whileincluding the longitudinal direction. Next, from depths of 0.02 mm, 0.05mm, and 0.09 mm from the surface of the notch bottom, samples where aregion of 7 μm×7 μm could be observed were extracted by FIB processing,and thin film samples having a thickness of 100 nm or more and 300 nm orless were prepared. Thereafter, element mapping of Al and N of the thinfilm samples was performed in a range of 7 μm×7 μm using a fieldemission transmission electron microscope and EDS (energy dispersiveX-ray analysis). In the place where AlN was precipitated, the number ofAl and N detected was significantly higher than that in places where AlNwas not precipitated. Therefore, a region where the number of Al and Ndetected is high was determined as AlN, and the number of AlN regionshaving an equivalent circle diameter of 50 nm or more and 100 nm or lesswas counted. By dividing the number by the observation area, the numberdensity of AlN at each depth position was obtained. By averaging thevalues, the number density of AlN included in the region from thesurface to a depth of 0.1 mm was obtained.

Using the test piece after the Charpy impact test, the number density ofTi-based precipitates having an equivalent circle diameter of 5 to 50 nmcontained in the region from the surface of the sample to a depth of 0.1mm was measured. The measurement method is as follows. First, in orderto obtain the cross section of the notch bottom, the test piece was cutin a cross section which was perpendicular to the notch while includingthe longitudinal direction. Next, from points at depths of 0.02 mm, 0.05mm, and 0.09 mm from the surface of the notch bottom, samples where aregion of 7 μm×7 μm could be observed were extracted by FIB processing,and thin film samples having a thickness of 100 nm or more and 300 nm orless were prepared. Thereafter, EDS analysis of phases other than Fe,which had an equivalent circle diameter of 5 to 50 nm, was performed byobserving the region of 7 μm×7 μm at a magnification of 200,000-foldusing a HAADF-STEM method with the field emission transmission electronmicroscope, and the number of phases where Ti was detected among thephases was counted. The value obtained by dividing the above-describednumber by the observation area was used as the number density ofTi-based precipitates having an equivalent circle diameter of 5 to 50 nmat each depth position and the average of the values was used as thenumber density of the Ti-based precipitates having an equivalent circlediameter of 5 to 50 nm contained in the region from the surface to adepth of 0.1 mm.

Using the test piece after the Charpy impact test, the grain size ofprior austenite grains at a position 0.05 mm from the surface wasmeasured. Specifically, the prior austenite grain boundaries wererevealed by using a corrosive liquid containing picric acid andhydrochloric acid with respect to the cross section of the notch bottomdescribed above, and according to the comparison method defined in JIS-G0551, the grain sizes of five points were obtained. By averaging thegrain sizes, the grain size of the prior austenite grains was evaluated.

In order to evaluate the hot ductility of the steel ingot, a test piecehaving a diameter of 10 mm and a length of 120 mm was cut out from thesteel ingot. This test piece was heated to 1350° C. in a vacuumatmosphere of 10⁻¹ to 10⁻² Pa through electric heating, retained for 1minute, and then cooled to 800° C. at 1° C./s. After conducting atensile test at a strain rate of 0.005 s⁻¹, the diameter of the finalfractured portion was measured, a reduction in the area was calculated,and this was used as an index of hot ductility. A sample having a hotductility of 50% or more was determined to have good hot ductility.

Table 3 shows the hot ductility of each sample, the amount of surfacelayer AlN after carburizing (that is, the number density of AlNcontained in the region from the surface to a depth of 0.1 mm), theamount of the surface layer Ti-based precipitates after carburizing(that is, the number density of the Ti-based precipitates contained inthe region from the surface to a depth of 0.1 mm), the grain size, andthe impact value ratio.

TABLE 1 Composition (mass %) * The remainder includes Fe and impurities.Steel No. C Si Mn P S Cr O N Ti Nb B Al 1 0.21 0.10 1.12 0.020 0.0151.10 0.002 0.005 0.035 0.028 0.0019 0.004 2 0.27 0.21 0.41 0.014 0.0251.78 0.002 0.009 0.170 0.011 0.0026 0.002 3 0.16 0.32 0.85 0.005 0.0431.03 0.001 0.003 0.020 0.023 0.0020 0.006 4 0.20 0.31 0.50 0.012 0.0211.59 0.002 0.005 0.152 0.013 0.0015 0.005 5 0.22 0.32 0.75 0.014 0.0131.13 0.001 0.006 0.046 0.030 0.0025 0.004 6 0.20 0.15 1.45 0.014 0.0150.32 0.001 0.006 0.035 0.025 0.0020 0.006 7 0.21 0.23 0.65 0.013 0.0111.21 0.001 0.005 0.060 0.018 0.0032 0.005 8 0.20 0.28 0.82 0.017 0.0131.08 0.001 0.005 0.039 0.030 0.0020 0.005 9 0.22 0.20 0.38 0.013 0.0151.83 0.002 0.004 0.049 0.025 0.0017 0.004 10 0.22 0.26 0.71 0.012 0.0181.37 0.001 0.005 0.045 0.031 0.0023 0.008 11 0.23 0.19 0.55 0.013 0.0151.45 0.002 0.006 0.031 0.031 0.0019 0.004 12 0.21 0.25 0.63 0.010 0.0131.22 0.001 0.004 0.038 0.032 0.0015 0.005 13 0.19 0.23 0.82 0.015 0.0140.99 0.002 0.006 0.033 0.015 0.0025 0.003 14 0.16 0.15 0.43 0.015 0.0132.11 0.001 0.007 0.051 0.015 0.0021 0.007 15 0.20 0.24 0.79 0.020 0.0181.06 0.003 0.014 — — — 0.032 16 0.21 0.21 0.45 0.020 0.020 0.96 0.0030.008 0.031 0.015 0.0013 0.030 17 0.19 0.25 0.66 0.015 0.013 1.22 0.0010.006 0.025 0.015 0.0025 0.011 18 0.19 0.25 0.70 0.023 0.022 1.10 0.0020.008 0.012 0.031 0.0020 0.005 19 0.21 0.20 0.75 0.019 0.019 1.05 0.0020.009 0.018 0.009 0.0019 0.006 20 0.21 0.34 0.52 0.012 0.023 1.61 0.0020.005 0.145 0.013 — 0.005 21 0.24 1.23 0.35 0.014 0.015 1.32 0.002 0.0090.160 0.012 0.0022 0.003

TABLE 2 Composition (mass %) * The remainder includes Fe and impurities.Rare earth Steel No. Ni Mo Cu Co W V Pb Ca Mg Zr Te metal 1 — — — — — —— — — — — — 2 — — — — — — — — — — — — 3 1.61 — — — — — — — — — — — 4 —0.29 — — — — — — — — — — 5 — — 0.20 — — — — — — — — — 6 — — — 0.22 — — —— — — — — 7 — — — — 0.31 — — — — — — — 8 — — — — — 0.11 — — — — — — 9 —— — — — — 0.05 — — — — — 10 — — — — — — — 0.0012 — — — — 11 — — — — — —— — 0.0011 — — — 12 — — — — — — — — — 0.0021 — — 13 — — — — — — — — — —0.0010 — 14 — — — — — — — — — — — 0.0031 15 — — — — — — — — — — — — 16 —— — — — — — — — — — — 17 — — — — — — — — — — — — 18 — — — — — — — — — —— — 19 — — — — — — — — — — — — 20 — — — — — — — — — — — — 21 — — — — — —— — — — — —

TABLE 3 Amount of surface Amount of surface layer Ti-based Temperaturelayer AlN after precipitates after Impact Test Steel rising ratecarburizing carburizing Grain value No. No. Classification Hot ductility(° C./min) (pieces/μm²) (pieces/μm²) size ratio 1 1 Invention 50% ormore 20 0.0 2.5 9.4 1.28 2 2 Example 50% or more 45 0.0 10.0 9.7 1.32 33 50% or more 20 0.1 1.1 9.1 1.25 4 4 50% or more 20 0.1 3.4 9.9 1.31 55 50% or more 10 0.1 4.7 9.6 1.31 6 6 50% or more 20 0.1 1.5 9.3 1.28 77 50% or more 20 0.1 2.7 9.5 1.30 8 8 50% or more 20 0.1 1.8 9.5 1.29 99 50% or more 20 0.0 3.8 9.5 1.29 10 10 50% or more 15 0.2 3.6 9.6 1.3111 11 50% or more 20 0.0 2.0 9.4 1.28 12 12 50% or more 15 0.1 3.3 9.51.29 13 13 50% or more 20 0.0 3.1 9.0 1.25 14 14 50% or more 30 0.0 2.59.1 1.26 23 21 50% or more 45 0.0 8.3 9.5 1.30 15 15 Comparative 50% ormore 20 2.0 0.0 7.2 1.00 16 16 Example 50% or more 20 1.8 3.5 8.1 1.0817 17 50% or more 25 0.6 3.6 7.3 1.15 18 18 30% 20 0.1 0.0 7.5 1.10 1919 50% or more 35 0.0 0.0 6.7 0.98 20 13 50% or more 1 7.2 2.6 6.5 0.9621 13 50% or more 60 0.0 0.4 6.5 1.01 22 20 50% or more 20 0.1 3.4 9.81.10

In Test Nos. 1 to 14 and 23, which are invention examples, in thesurface region, the number densities of AlN and Ti-based precipitateshaving predetermined sizes were in appropriate ranges, the grains wererefined, and thus good impact resistance was provided. In addition, thehot ductility was also good.

Contrary to this, Test Nos. 15 to 22 as comparative examples did nothave preferable properties.

In Test No. 15, the Al content and the N content of the base metal wereexcessive, and AlN precipitated excessively during carburizing heating.Furthermore, since Test No. 15 did not contain Ti and Nb, Ti-basedprecipitates were not generated. Moreover, Test No. 15 did not containB. Accordingly, Test No. 15 had a lower impact value than those of theexamples. In Test Nos. 16 and 17, the Al content of the base metal wasexcessive and AlN precipitated excessively during carburizing heating.Therefore, the impact values thereof were lower than those of theexamples.

In Test No. 18, since the Ti content of the base metal was insufficient,a reduction in hot ductility due to Nb was not suppressed: and only lowmanufacturability was achieved, which was inappropriate. In addition, inTest No. 18, since the Ti content of the base metal was insufficient,Ti-based precipitates were not generated, and grains became coarse.Therefore, the impact value was lower than those of the examples.

In Test No. 19, although Ti was deficient, the Nb content was small.Therefore, a reduction in hot ductility did not occur. However, in TestNo. 19, Ti-based precipitates were not generated due to the deficiencyof Ti and grains became coarse. Therefore, the impact value was lowerthan those of the examples.

In Test No. 20, since the temperature rising rate during carburizing wasinsufficient and the amount of precipitated AlN increased, grains becamecoarse, and the impact value was lower than those of the examples.

In Test No. 21, since the temperature rising rate during carburizing wastoo high, the amount of N introduced during temperature rising wassmall, the amount of Ti-based precipitates was small. As a result,grains became enlarged, and the impact value was lower than those of theexamples.

Test No. 22 did not contain B. Accordingly, Test No. 22 had a lowerimpact value than those of the examples.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide acase-hardened steel component having excellent impact resistance. Thecase-hardened steel component causes a reduction in the amount ofmaterials used for a component for machine structural use andcontributes to a reduction in the weight the component for machinestructural use. Therefore, the industrial effect of the presentinvention is extremely large.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   1: CARBURIZED LAYER    -   2: Ti-BASED PRECIPITATE    -   3: COARSE AlN

1. A case-hardened steel component comprising: a base metal; and acarburized layer, wherein a chemical composition of the base metalincludes, by mass %, C: 0.16% to 0.30%, Si: 0.10% to 2.00%, Mn: 0.30% to2.00%, Cr: 0.20% to 3.00%, S: 0.001% to 0.100%, N: 0.003% to 0.010%, Ti:0.020% to 0.200%, Nb: 0.010% or more and less than 0.050%, B: 0.0005% to0.0050%, Ni: 0% to 3.00%, Mo: 0% to 0.80%, Cu: 0% to 1.00%, Co: 0% to3.00%, W: 0% to 1.00%, V: 0% to 0.30%, Pb: 0% to 0.50%, Ca: 0% to0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0500%, Te: 0% to 0.1000%, a rareearth metal: 0% to 0.0050%, Al: limited to 0.010% or less, O: limited to0.0050% or less, P: limited to 0.025% or less, and Fe and impurities asa remainder, a number density of Ti-based precipitates having anequivalent circle diameter of 5 to 50 nm in a surface region from asurface to a depth of 0.1 mm is 0.5 pieces/μm² or more, and in thesurface region, a number density of AlN having an equivalent circlediameter of 50 nm or more and 100 nm or less is 0.5 pieces/μm² or less.2. The case-hardened steel component according to claim 1, wherein thechemical composition of the base metal includes, by mass %, one or twoor more selected from the group consisting of Ni: more than 0% and 3.00%or less, Mo: more than 0% and 0.80% or less, Cu: more than 0% and 1.00%or less, Co: more than 0% and 3.00% or less, W: more than 0% and 1.00%or less, and V: more than 0% and 0.30% or less.
 3. The case-hardenedsteel component according to claim 1, wherein the chemical compositionof the base metal includes, by mass %, one or two or more selected fromthe group consisting of Pb: more than 0% and 0.50% or less, Ca: morethan 0% and 0.0100% or less, Mg: more than 0% and 0.0100% or less, Zr:more than 0% and 0.0500% or less, Te: more than 0% and 0.1000% or less,and a rare earth metal: more than 0% and 0.0050% or less.
 4. Thecase-hardened steel component according to claim 2, wherein the chemicalcomposition of the base metal includes, by mass %, one or two or moreselected from the group consisting of Pb: more than 0% and 0.50% orless, Ca: more than 0% and 0.0100% or less, Mg: more than 0% and 0.0100%or less, Zr: more than 0% and 0.0500% or less, Te: more than 0% and0.1000% or less, and a rare earth metal: more than 0% and 0.0050% orless.